Conventionally, a carriage which is provided with, for example, two scanning optical sections is arranged in an image reading apparatus to read a document placed on an original table glass (hereinafter referred to as a fixed document). The first scanning optical section of the carriage illuminates the document through the original table glass with a document illuminating light source, and irradiates the reflecting mirror of the second scanning optical section with the reflection light from the document through a reflecting mirror. The second scanning optical section moves in a vertical scanning section at the speed of V and the first scanning optical section moves in the same direction at the speed of ½V. The reflection light from the document which entered the second scanning optical section is read by a CCD line sensor through another reflecting mirror.
On the other hand, in an image reading apparatus provided with an automatic document feeder (ADC), a carriage moves from a standby position to a reading position where the document is automatically fed.
The standby position of the carriage is selected such that a black shading processing and a white shading processing serving as an image reading standard in a shading correction are carried out at a stable position.
In this way, in the case where the standby position of the carriage is set to carry out black shading processing and white shading processing preferentially, the disadvantage is pointed out when the carriage is moved to a document reading position after a shading processing. For example, to determine the size of a fixed document and to read a document using a CCD line sensor, the carriage is moved from the standby position to a document reading position to detect the length of a sheet through shading processing according to whether or not there is reflection light of the sheet. Then, the carriage is returned to the original standby position after the shading processing is ended and then moved towards an image reading position along the vertical scanning direction until it reaches a given position to start to read an image.
Thus, in a case of reading an image of a fixed document, the carriage is first moved from the standby position to an image reading position to carry out shading processing and then returned to the original standby position after the shading processing. In a case where returns to the standby position, the carriage vibrates at the standby position because it stops suddenly. Then, the carriage is moved to read an image after the convergence time, i.e., the time required for convergence of the vibration of the carriage at the standby position elapses. Thus, the convergence time of the vibration along with the movement direction switching of the carriage needs to be guaranteed, which becomes a cause of the deterioration of image reading time.